Evidently, the folk of Laketown believed that cutting the bridge would either offer protection from Smaug landing, or as davem stated earlier, that cutting the bridge would diffuse the stampeding masses, rather than centralize their egress along one route.
In any case, Smaug did not land, and there was no crush of hysterical refugees flooding the bridge.
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And your little sister's immaculate virginity wings away on the bony shoulders of a young horse named George who stole surreptitiously into her geography revision.
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